The Research Of Interference In The Circuit Produced By Distributed Capacitance Of Electronic Transformer

The 21st century is the age of information, the rapid development of information technology makes people rely more and more on electronic equipment that not run without power supply. Switched-mode power supply(SMPS) have advantages about efficiency, volume and weight compared with the linear power supply, especially the high frequency SMPS is becoming lighter, smaller, more efficient and more reliable, so it has been widely used. Electronic transformer is the core component of SMPS and its function is to achieve energy conversion and transmission, as the high frequency of SMPS, effect of distributed parameter on performance of power supply system is becoming more important in electronic transformer. At present the study of high-frequency transformer, the more concentrated in the loss at high frequency characteristics,leakage inductance characteristics, optimization design and application research of the transformer. But stray capacitance is seldom being considered in transformer design. The function of transformer mainly to achieve energy storage and transmission, isolation and voltage transformation in SMPS. In the high frequency, the stray capacitance of transformer can’t be ignored, the stray capacitance is not only impact switch loss, high frequency impedance characteristics, but also provide the coupling channel for EMI noise, so result in serious electromagnetic interference. With the gradual strict requirements for electromagnetic interference, so the study of parasitic capacitance in high frequency electronic transformer is of great and practical significance.The object of study is flyback converter in this paper. First, the equivalent circuit model of electronic transformer are analyzed based on the two-port network theory, circuit theory and electromagnetic field theory, and analyze the effect of distributed parameter on electrical system in high-frequency transformer. Second, based on the PExprt/PEmag software, design different winding strategy model of transformer, then the values of distributed capacitances of different winding strategy have been obtained by finite element software Ansoft Maxwell, according to the result that compare extracted parameters to the results of theoretical analysis, so the influence of different winding strategy on winding distributed capacitances and the rationality of this kind of analysis method are validated. Finally, under the effect of high frequency switch voltage, a common-mode conducted EMI model of a flyback converter is built after confirm the distributed capacitance between the primary and secondary windings of the transformer is a important coupling channel of common-mode noise. Based on the model, more attention is paid on the effect of winding strategy and different ways of connection of transformer with external circuit on common-mode EMI. Finally, in order to reduce common-mode noise, a common-mode filter is introduced. The above analysis is verified by Pspice software.